Given an array of size n, Find the subarray with least average of size k.
 

Example 1:

Input: nums = [30, 20, 10], k = 1
Output: 3
Explanation: Subarrays of length 1 are
{30}, {20}, {10}. {10} has the least 
average.

Example 2:

Input: nums = [30, 20, 10], k = 2
Output: 2
Explanation: Subarrays of length 2 are
{30, 20}, {20, 10}. {20, 10} has the least 
average.

Your Task:
You don't need to read or print anything. Yous task is to complete the function least_average() which takes the array and k as input parameter and returns the index of the first element of the subarray of size k that has least average.
 

Expected Time Complexity: O(n)
Expected Space Compelxity: O(1)
 

Constraints:
1 <= k <= n <= 100000
1 <= elements of array <= 1000000